1. Field of the Invention
The invention is related to an ultrasonic flow measuring technology, and particularly, to an ultrasonic flow measuring method for measuring flow velocities on a plurality of fluid flowing sections and then computing a flow or flowrate, if ultrasonic transducers are mounted on a pipe that had been already arranged on a place.
2. Description of the Background
A general ultrasonic flow measuring method is based on the fundamental technical background as follows: an ultrasonic one channel flowmeter is designed to measure a flow velocity VD on a part of a fluid flow section, for example the inner diameter of a pipe, using an ultrasonic wave and multiply the flow velocity VD by a flow coefficient K along with a fluid section area S to calculate a flow. An ultrasonic multi-channel flow measuring method includes steps of measuring a flow velocity VD and flow velocities on chords divided into a plurality of sections, using an ultrasonic wave, to calculating an average flow velocity VS of a fluid flow section and multiplying VS by a section area to calculate a flow. Another method is known to measure an average horizontal flow velocity at a plurality of water depths in an open sluice in order to compute a flow.
Typical ultrasonic flow measuring methods and apparatuses there for are disclosed as follows:
U.S. Pat. No. 5,531,124 granted on Jul. 2, 1996
U.S. Pat. No. 4,646,575 granted on Jul. 25, 1987
U.S. Pat. No. 4,860,593 granted on Aug. 29, 1989
U.S. Pat. No. 5,780,747 granted on Jul. 14, 1998
U.S. Pat. No. 4,676,321 granted on Jul. 25, 1996
Russian Pat. No. 2,138,782 granted on Sep. 27, 1999
The ultrasonic flow measuring methods already known have common properties as follows:
1) A flow measuring section is selected to be a section S in a right angle to a direction of a fluid flow. In case of a pipe, a section rectangular to a centerline is selected.
2) Therefore, a flow velocity in a right angle direction to a section to be firstly measured by an ultrasonic wave is calculated. At that time, it is assumed that the direction of the flow velocity is corresponded to a fluid flow direction.
3) An ultrasonic flow velocity measuring method includes a frequency difference method and a phase difference method, but these methods are based on transit time difference method, which has been broadly used.
A typical transit time difference flow velocity measuring expression is as follows:                     V        =                                                                              L                  2                                                  2                  ⁢                  d                                            ⁢                              t                2                                      -                                          t                1                                                              t                  1                                ⁢                                  t                  2                                                              =                                                    L                2                                            2                ⁢                d                                      ⁢                          xe2x80x83                        ⁢                                          Δ                ⁢                                  xe2x80x83                                ⁢                t                                                              t                  1                                ⁢                                  t                  2                                                                                        (        1        )            
Wherein, L is an interval distance between paired transducers 1 and 2, d is a projection distance of L in which d=Lcos"PHgr", t1 is a transit time in a flow velocity direction from the paired transducer 1 to the paired transducer 2 and t2 is a transit time in a direction contrary to a flow velocity from the paired transducer 2 to the paired transducer 1 (referring to FIG. 1).
A flow computing expression of an ultrasonic one-channel flow computing method is as follows:
Q=Kxc2x7VDxc2x7Sxe2x80x83xe2x80x83(2)
Wherein, K is a flow coefficient, VD is a flow velocity on a diametric line to be measured by the expression (1) and S is a section area of fluid as defined above, for example an inner section area of a pipe.
One of flow calculation expressions for an ultrasonic multi-channel flow measuring method is as follows:
Q=VSxc2x7Sxe2x80x83xe2x80x83(3)
Wherein, VS is a total average flow velocity on a plurality of chords to be measured by the expression (1).
An ultrasonic flowmeter has most characteristics as follows: unlike another flowmeter, mounting transducers on a pipe that had been already arranged in a place can perform a flow measurement. Even under the condition that fluid is transported through the pipe, the transducers can be mounted on the pipe through the drilling work thanks to the technology progress. For the characteristics, the ultrasonic flowmeter is very often used.
Particularly, the ultrasonic multi-channel flow measuring method can measure a flow, exactly, even if a condition that K=constant, for example a distance of a straight portion of a pipe becomes at least 25D and Re greater than 104, is not secured and a flow velocity distribution is not a normal state, or if the inner diameter of the pipe is relatively larger. Therefore, the characteristics enable the ultrasonic flowmeter to be used as a flowmeter for a larger pipe.
FIG. 2 shows five chords for measuring a flow velocity, but the number of chord can be increased as requested. As shown in FIG. 2, in order that d=Lixc2x7cos"PHgr"i=const, mounting angles "PHgr"i of paired transducers 1i and 2i are not equal to each another.
As represented in the expressions (2) and (3), a flow measuring error xcex4Q is considered as a sum of a flow velocity measuring error xcex4V and a section area measuring error xcex4S. The flow measuring error xcex4Q in the ultrasonic one-channel flow measuring method is as follows:
xcex4Q=xcex4K+xcex4VD+xcex4Sxe2x80x83xe2x80x83(4)
The flow measuring error xcex4Q in the ultrasonic multi-channel flow measuring method is as follows:
xcex4Q=xcex4Vi+xcex4M+xcex4Sxe2x80x83xe2x80x83(5)
Wherein, xcex4K is a flow coefficient error, xcex4M is an error followed by calculating an average flow velocity of a section using a flow velocity Vi measured on a plurality of chords, for example an approximate integral error of an expression that       V    S    =            1              2        ⁢        R              ⁢                  ∫                  -          R                          +          R                    ⁢                        V          ⁢                      (            r            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          r                    
In the expressions (4) and (5), the flow measuring error xcex4Q is determined by the flow velocity measuring error xcex4V and the section area measuring error xcex4S. Therefore, in order to enhance the accuracy of the flow measuring, the flow velocity measuring error xcex4V and the section area measuring error xcex4S are significantly reduced. In the flow velocity measuring expression (1), assuming that the transit time measuring errors includes an accidental error component, a flow velocity measuring error is as follows:
xcex4=(2xcex4L+xcex4d)+{square root over (xcex42t1+xcex42t2+xcex42xcex94t)}=(2xcex4L+xcex4d)+Axe2x80x83xe2x80x83(6)
A={square root over (xcex42t1+xcex42t2+xcex42xcex94t)}
Wherein, xcex4L is a measuring error of an interval distance L, and xcex4d is a measuring error of d, in which L and d are a constant to be inputted into an arithmetic logic processor or microprocessor after being measured. Therefore, the symbols of the xcex4L and xcex4d are not changed. In other words, these errors are a fixing error. xcex4t1, xcex4t2 and xcex4xcex94t are errors of each of transit times t1 and t2, and the error xcex94t=t2xe2x88x92t2.
As represented in the expression (6), even through t1 and t2 are precisely measured under the condition that A is reduced enough to be ignored, if xcex4L and xcex4d are relatively larger, the flow velocity measuring error xcex4V becomes larger. Herein, what the measuring error xcex4L of L is multiplied by 2 is because of L2. In case of the pipe, the section area S is calculated by measuring ab inner diameter D as follows:   S  =            π      ⁢              xe2x80x83            ⁢              D        2              4  
The calculation error of the section area is as follows:
xcex4S=2xcex4Dxe2x80x83xe2x80x83(7)
Wherein, xcex4D is a measuring error of an inner diameter D.
Therefore, the measuring errors of geometrical integers or constants L, d, D appear as a flow measuring error as follows:
xcex4Q=(2xcex4L+xcex4d+2xcex4D)+Axe2x80x83xe2x80x83(8)
These errors are a fixing error represented as an arithmetical sum with their symbols being known.
In case of a flowmeter of a flange type, the inner diameter D is measured several times to obtain its average value {overscore (D)}, so xcex4S=2xcex4D is secured to become smaller. But, measuring the interval distance Li between the transducers is not simple. There is a measuring instrument capable of measuring an inner diameter, exactly, but there is not a precise measuring instrument that can directly measure an ultrasonic transit distance Li between the transducers disposed at an angle "PHgr" to an axis of a pipe. For it, it is very difficult to secure a small value of xcex4Li enough to be ignored. A measuring error xcex4d of the projection distance d=Lcosxcex4 calculated by measuring the interval distance L and a mounting angle "PHgr" of the transducer is as follows:
xcex4d=xcex4L+xcex4cos"PHgr"xe2x80x83xe2x80x83(9)
Herein,   δcosϕ  =                    cos        ⁢                  (                      ϕ            ⁢                          +              _                        ⁢            α                    )                            cos        ⁢                  xe2x80x83                ⁢        ϕ              =                  cos        ⁢                  xe2x80x83                ⁢        α            ⁢              +        _            ⁢              tan        ⁢                  xe2x80x83                ⁢        δsinα            -      1      
Therefore, xcex4d is as follows:
xcex4dMAX=xcex4L+(cosxcex1+tan"PHgr"sinxcex1xe2x88x921)xe2x80x83xe2x80x83(10)
Wherein, xcex1 is a absolute error of an angle "PHgr" to be measured, for example if xcex4=45xc2x0 and xcex1=0.25xc2x0, xcex4cos"PHgr"≈0.44%. The geometrical distance measuring error xcex4h is as follows:
xcex4h=2xcex4L+xcex4d+2xcex4Dxe2x89xa13xcex4L+2xcex4Dxe2x80x83xe2x80x83(11)
But, if the transducers are mounted on the pipe that had already been arranged at a place, the inner diameter D of the pipe cannot be measured at firsthand. Furthermore, the inner diameter identified by a pipe manufacturer has a predetermined deviation. If an anti-corrosion layer is coated, its thickness cannot be identified. Due to it, it is common that the absolute error of the inner diameter is approximately 2xcx9c4 mm. If xcex94D=4 mm, xcex4D=4xc3x97100/600xe2x89xa10.67%, and the section error xcex4S=2xc3x970.67=1.34%.
On the other hand, there discloses a method of exactly measuring a transit distance Li using an ultrasonic wave. A sound velocity C in fluid of a pipe is measured by a three-point method, and then the transit time t1xc2x72 between paired transducers is measured, so L=Cxc3x97t1xc2x72, which suggests the exact value of L. For example, a method which is disclosed in U.S. Pat. No. 5,531,124 issued on Jul. 2, 1996 comprises steps of measuring the transit time t1xc2x72 between paired transducers, inserting one transducer into the pipe by xcex94L and again measuring the transit time txcex94, thereby measuring a flow velocity on the inner diameter of a pipe.                                           t            1.2                    =                      L            C                          ;                  xe2x80x83                ⁢                              t            Δ                    =                                    L              -                              Δ                ⁢                                  xe2x80x83                                ⁢                L                                      C                          ;                  xe2x80x83                ⁢                                            t              1.2                        -                          t              Δ                                =                                                                      Δ                  ⁢                                      xe2x80x83                                    ⁢                  L                                C                            ⁢                              
                            ∵              C                        =                                          Δ                ⁢                                  xe2x80x83                                ⁢                L                                                              t                  1.2                                -                                  t                  Δ                                                                                        (        12        )            xe2x80x83Li=C(t1xc2x72)ixe2x80x83xe2x80x83(13)
If the transit time t1xc2x72 and the distance xcex94L are very precisely measured, the error of Li gets smaller. On the contrary, if the inner diameter is larger, the error of Li obtained by the expressions (12) and (13) may become larger. The reason is as follows: the sound velocity C obtained by the expression (12) is a sound velocity in an interval of xcex94L, but it may be not equal to the sound velocity in the interval xcex94L. In other words, if a fluid temperature of the interval xcex94L away in a predetermined distance from a pipe wall is not corresponded to an average temperature of all intervals Li, the sound velocity C obtained by the expression (12) is not the same as a sound velocity CLi in the interval Li. If xcex94L=Li/2, C is equal to CLi. But, if the inner diameter of the pipe is larger, the length of the transducer for measuring the sound velocity is extended, because Li becomes larger.
A main object of the invention is to provide to an ultrasonic flow measuring method for measuring flow velocities on a plurality of fluid flowing sections and then computing a flow or flowrate, if ultrasonic transducers are mounted on a pipe that had been already arranged on a place.
Another object of the invention is to provide an ultrasonic flow measuring method for significantly reducing an error component of geometrical integers necessary for measuring and calculating a flow velocity and a flowrate.
Another object of the invention is to provide an ultrasonic flow measuring method for enabling the same mounting angle of each of paired transducers to facilitate the transducers to be mounted on a pipe that was already arranged on a place.
According to the invention, an ultrasonic flow measuring method comprises steps of selecting an inner section area S"PHgr" of a pipe cut at an angle xcex4 of 45xc2x0 as a flow measuring section, in which the inner section area S"PHgr" is an ellipse or oval form, mounting paired transducers at two points having a longer diameter of the inner section area S"PHgr", mounting a predetermined number of paired transducers along the periphery of the ellipse on both sides by the center of the longer diameter, measuring flow velocities on a plurality of chords of the ellipse using an ultrasonic wave, computing an average flow velocity of the section area S"PHgr" and multiplying the average flow velocity by the section area S"PHgr" to measuring the flow or flowrate, in which the longer diameter of the section area S"PHgr" is subject to being measured using the ultrasonic wave.